Several processes are known in the art for growing crystal ingots of semi-conductor materials for use in fabricating integrated circuits and photovoltaic devices such as solar cells. Batch and continuous Czochralski (“CZ”) processes are widely used for semiconductor materials such as silicon, germanium, or gallium arsenide doped with an elemental additive such as phosphorus (n-type dopant) or boron (p-type dopant) to control the resistivity of the crystal. These processes are generally summarized as follows. A heated crucible holds a melted form of a charge material from which the crystal is to be grown. A seed is placed at the end of a cable or rod that will enable the seed to be lowered into the melt material and then raised back out of the melt material. When the seed is lowered into the melt material, it causes a local decrease in melt temperature, which results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed is slowly withdrawn from the melt material. As the seed is withdrawn or pulled from the melt material, the portion of the newly formed crystal that remains within the melt material essentially acts as an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn or pulled from the melt material, resulting in crystal ingot growth as the seed is continually raised.
In batch CZ, the entire amount of charge material (semi-conductor and dopant) required for growing a single crystal ingot is melted at the beginning of the process. In continuous CZ (“CCZ”), the charge material is continually or periodically replenished during the growth process. In CCZ, the growth process may be stopped at intervals between crystal growth to harvest the crystal or may continue without stopping between crystal growth.
The batch CZ process is typically carried out using a pulling apparatus comprising a gas chamber, a quartz crucible positioned inside the chamber, semiconductor charge material and dopant loaded into the crucible, a heater for melting the charge material, and a pulling mechanism for pulling or drawing up a single crystal ingot of the doped semiconductor material. To carry out the CCZ process, it is necessary to modify the traditional apparatus to include a means for feeding additional charge material to the melt in a continuous or semi-continuous fashion. In an effort to reduce the adverse effects of this replenishing activity on simultaneous crystal growth, the traditional quartz crucible is modified to provide an outer or annular melt zone (into which the semi-conductor is added and melted) and an inner growth zone (from which the crystal is pulled). These zones are in fluid flow communication with one another.
In general, it is desirable for the dopant concentration in the crystal ingot to be uniform both axially (longitudinally) and radially. This is difficult to achieve due, in part, to segregation. Segregation is the tendency of the impurity or dopant to remain in the melt material instead of being drawn-up into the crystal ingot. Each dopant has a characteristic segregation coefficient that relates to the comparative ease with which the dopant atom can be accommodated into the ingot's crystal lattice. For example, because most dopant atoms do not fit into the silicon crystal lattice as well as a silicon atom, dopant atoms typically are incorporated into the crystal at less than their proportional concentration in the melt, i.e., dopants in a silicon melt generally have a segregation coefficient of less than 1. After the doped silicon is melted and crystal growth has begun, the dopant concentration increases in the melt due to rejection of the dopant at the crystal growth interface.
In general, the dopant concentration of the pulled single crystal is given as kC where the dopant concentration in the molten polycrystalline or raw material is C and where k is a segregation coefficient that is typically less than 1. During a doped batch CZ process, the amount of melt material in the crucible decreases as the crystal ingot grows, and as a result of segregation, the dopant concentration gradually increases in the remaining melt material. Due to the higher dopant concentration in the melt material, the dopant concentration in the crystal ingot also becomes higher, resulting in varying resistivity along the radial and longitudinal axis of the crystal. A doped batch CZ process potentially results in an ingot having the desired resistivity in only a small portion of the ingot.
It has been suggested that more uniform resistivity may be obtained using a CCZ process where the dopant concentration in the raw material fed successively into the annular melt zone is made equal to the dopant concentration in the pulled single crystal and the amount of single crystal pulled per unit time is made equal to the amount of charge material supplied. In so doing, it is intended that the amount of dopant supplied and pulled are balanced with each other so that the dopant concentration in the inner crucible equals C/k and the concentration in the outer crucible equals C in a steady state. A variety of different processes and configurations of crucibles have been suggested in an effort to maintain the relative concentrations of the dopant within the inner and outer zones of the crucible and to otherwise achieve uniform resistivity. One problem that continues to persist during a CCZ run is the tendency for dopant to migrate or diffuse to the outer melt zone of the crucible (due to the concentration gradient), which results in lower dopant concentration and higher resistivity at the seed end of the next crystal ingot until the steady state can be achieved again.
In the past, boron has traditionally been used as the dopant for silicon single crystals used in photovoltaic solar cell applications. It has been recognized, however, that boron forms recombination active defects with oxygen under illumination thereby lowering the minority carrier lifetime. This effect known as “light induced degradation” or “LID” causes a significant voltage and current drop of the solar cells when in operation. See, J. Schmidt, A. G. Aberle and R. Hezel, “Investigation of carrier lifetime instabilities in Cz-grown silicon,” Proc. 26th IEEE PVSC, p. 13 (1997); S. Glunz, S. Rein, J. Lee and W. Warta, “minority carrier lifetime degradation in boron-doped Czochralski silicon,” J. Appl. Phys., 90, pp. 2397 (2001). This problem can be circumvented by using low-oxygen material or high-resistivity material to minimize boron content; however, it is also known that higher efficiencies can be obtained using relatively low-resistivity material (around 1.0 Ω-cm or below). Low-resistivity material requires a higher dopant concentration.
It has been suggested that boron can be replaced by gallium, which shows similar electronic behavior in the silicon band structure but does not form recombination active defects under illumination. While it has been suggested that a gallium doped silicon single crystal can be produced via a batch CZ process, gallium has a much smaller segregation coefficient than boron, which means the batch CZ process results in a gallium doped crystal that exhibits a large axial resistivity variation. This lack of uniformity increases the cost of production due to the limited amount of acceptable material in each ingot and/or the cost of development of cell manufacturing processes that can accommodate material exhibiting a wide resistivity range. For this reason, the use of gallium doped crystals for solar cell applications has not been widely adopted in an industrial setting although the advantages of gallium doped silicon wafers in terms of LID reduction has been known for decades.
The use of CCZ has not been suggested for making ingots doped with gallium, aluminum, or indium, all of which have a small silicon segregation coefficient. This is likely due to the fact that elemental gallium (the most preferred of the three dopants) would be difficult to add in a sufficiently high concentration using a continuous or semi-continuous feeding apparatus because it melts near room temperature and would stick to the apparatus. This not only has the potential of damaging the apparatus, but also creates operational problems such as a lack of control of the actual amount of gallium being added to the melt. In addition, gallium forms a highly volatile suboxide (Ga2O) that results in significant loss of gallium from the melt due to evaporation. This evaporation effect would be exacerbated in a CCZ system due to the longer run times and greater melt surface area associated with CCZ.